A system for indicating the ambient air temperature outside of a vehicle typically has a temperature sensor mounted outside of the vehicle, a temperature indicator inside, and a processing means somewhere in between. The temperature signals from the sensor are usually processed in digital format. In order for an accurate ambient temperature reading to be indicated, the system should account for distortions in the temperature sensor measurement due to extraneous heat sources. Some significant sources of thermal distortion are heat conducted from the motor vehicle itself, radiant heat from the ground, and the thermal load from the sun. These distortions in the measured temperature are especially great when the vehicle is either moving at a slow rate of speed or is totally stopped.
The effects of heat conducted from the motor vehicle can be lessened by proper placement of the sensor on the vehicle (preferably towards the front) and appropriately designed insulation. Even with such measures, it is necessary to allow for heating of the sensor by the waste heat from the vehicle's engine, at least when the vehicle is at a standstill. Insulation, although retarding the heating of the sensor, nevertheless has the adverse effect that, for example when the vehicle leaves a garage, the sensor cannot cool from a higher temperature to a lower outside temperature as quickly as without the insulation.
In one solution to this problem, a specific system was developed for vehicles manufactured by Pontiac.TM. (Pontiac is a registered trademark of General Motors Corporation). In the Pontiac.TM. system, a vehicle speed sensor is additionally provided and, when the vehicle comes to a stop, the last temperature detected by the outside sensor is indicated inside of the vehicle, and temperature measurements taken during the standstill are ignored. Only after a fixed time delay after the motor vehicle drives on is the temperature last indicated erased and updated with a current temperature reading. The length of the delay time is selected so that the temperature sensor has time to "flush" itself of any heat absorbed during the period of standstill. This delay time can range up to ten minutes.
A disadvantage of a Pontiac-type system is that the delay can be significant in comparison to the time when the temperature sensor is able to indicate a true temperature. Since worst case conditions must be taken into account in determining the fixed delay time, the length of time before a current temperature is displayed is needlessly long. During the delay period, the indicator inside of the vehicle will be displaying (most likely) an erroneous temperature. The temperature measurements from the sensor are totally ignored during the delay, even though the measurements might be accurate.
In another prior art system, a variable update rate (the rate at which the indicated temperature is updated) is employed. If the vehicle has been stopped for less than a fixed amount of time (an hour, for example), the temperature indicating system goes into a slow update mode where the displayed temperature is only allowed to increment at a certain rate (e.g., one degree Celsius per minute). If the vehicle has been stopped for a period of time exceeding the fixed amount, the system goes into a fast update mode where the displayed temperature is the current sensor reading. The system remains in the fast update mode for a second fixed period of time (e.g., five minutes) and then reverts back into the slow update mode. This type of system suffers from the deficiency that the displayed temperature is typically higher than the actual temperature, since the system has not adequately taken into account the effects of the vehicle engine and ground heat.
A third prior art system is a variation on the variable update system except that it incorporated a further sophistication of using vehicle speed and actual engine temperature. If the vehicle is moving at less than a specific speed (e.g., 25 miles per hour), the engine temperature is compared to the current temperature from the outside sensor. If the engine temperature is greater than the outside temperature (as measured by the sensor), the system goes into the slow update mode described above. If the engine temperature is less than the outside temperature, the system enters the fast update mode, directly displaying the sensor temperature. Once the vehicle exceeds the specific speed (e.g., 25 mph) the system automatically switches into the fast update mode. The use of the engine temperature in this system does not determine what temperature to display, it merely aids in setting the proper update rate for the displayed temperature. Although this system attempted to further refine the temperature indication, it still typically indicated a higher temperature than the true outside ambient temperature.
It is, therefore, one object of the invention to provide, in a vehicle, an accurate indication of the outside ambient air temperature.
It is a further object to take into account, in the temperature indication, the effects of thermal distortions such as conducted vehicle heat and radiated ground heat.